Duplexers are usually devices found at the end of a system in items of radio equipment, i.e. antennas. They are designed to separate the signals transmitted from the signals received by the antenna. A duplexer conventionally includes two passband filters, one responsible for filtering the transmitted signals and the other responsible for filtering the received signals.
Conventionally, a passband filter for microwave signals includes a tunnel having a succession of compartments communicating with one another through a longitudinal passage, the dimensions and number of compartments being a function of the size and the center frequency of the passband of the filter. A duplexer for microwave signals therefore generally has two tunnels of this type that are respectively connected by one end to the transmission part and to the reception part of the radio equipment and that open jointly into the other end on the antenna side.
These passband filters are designed to meet the following conditions:
high resistance under temperature throughout the range; PA1 low loss in transmission and high return loss in the passband; and PA1 high rejection in the near band.
With respect to the rejection of the highest frequencies, especially for the elimination of the harmonics of the filtered signal, the equipment is generally provided with a low pass filter interposed between the duplexer and the antenna.
To meet the first condition, existing duplexers are generally made of a material that is highly stable under temperature, for example invar which is an alloy of iron and nickel with a coefficient of thermal expansion that is practically zero. However, this type of material proves to be very costly and very difficult to machine. Thus, the method generally used to manufacture duplexers is to make tunnels out of invar plates and solder transversal partition walls thereto so as to obtain compartments in these tunnels.
These duplexers are then methodically provided with a system of settings by screws to obtain the desired signal filtering and transmission characteristics. Tapped holes are made in the upper wall of the tunnels to receive setting screws. In general, one setting screw is provided per compartment with another screw being provided in the partition walls of each compartment in the longitudinal passage. The setting operation then includes adjusting the part of the screw that projects into the compartment or into the longitudinal passage. This operation proves to be very complicated and very lengthy.
Consequently, the present invention seeks to overcome the prior art drawbacks by proposing a duplexer that does not require a system of settings by screws for the usual frequencies.